In the manufacture of polystyrene foam sheet, rod, or other stock material, the method wherein heated and pressurized styrene resin is mixed with a foaming agent and then extruded to a low pressure zone is well known and widely used. The foam is thereafter subjected to a thermoforming step to make articles such as foam cups, sandwich containers, meat trays, etc. Some of the organic blowing agents employed have a solvating effect on the styrene resins. If the solvating effect is too great, deficiencies, such as sagging of the foam sheet, are encountered during subsequent thermoforming. In order to cope with this, extended aging has been found to be necessary. During this aging process a change takes place which results in an elimination or minimizing of the tendency of the polystyrene sheet to sag during thermoforming. It is believed that during aging the foam cells in the polymeric matrix become partially depleted of volatile hydrocarbons, which enter the atmosphere. It would be an advance in the state of the art if foamed polymer could be prepared without an excess of foaming agent. It also would be of considerable economic advantage if this could be accomplished with a system of low capital expense.
It is an object of the present invention to provide an extrusion process for utilizing a foaming agent maintained in gas form from injection through foam extrusion.
It is another object to provide an apparatus for utilizing a gas foaming agent and maintaining it in the gas form throughout the process. Yet another object of the present invention is to provide a process and apparatus utilizing precise control of the rate of introduction of a gas foaming agent into a foamable polymer.